The present invention relates to a blade arrangement with damping elements. The damping elements serve to dampen vibrations of the blade arrangement. The blade arrangement comprises a rotor and blades arranged on the circumference of the rotor, damping elements being loosely arranged between the blades and being in contact with the blades due to a centrifugal force, acting in the radial direction, during rotation of the rotor about a rotor axis.
Such blade arrangements are used in particular in fluid-flow machines, such as gas turbines. In this case, the individual blades generally consist of the blade body, a blade platform and the blade root, which is attached in corresponding recesses on the circumference of the rotor. During operation of the blade arrangement, undesirable flexural and torsional vibrations are produced by various excitation causes and may lead to premature material fatigue and thus to a shortened service life of the blade arrangement. The present invention relates to a blade arrangement with damping elements for damping these undesirable vibrations.
To reduce the vibrations of blade arrangements, damping elements which act between the individual blades are already used. As a rule, these damping elements are loose bodies which, in the state of rest, first of all lie between the blade roots of the blades on the rotor or on corresponding supporting structures and, during operation of the rotor, are pressed against the underside of the blade platforms of adjacent blades on account of the centrifugal force acting in the radial direction. In this case, each damping element is in contact with both adjacent blade platforms at the same time. As a result, the kinetic energy of a relative movement, caused by vibrations, between the blades can be converted into friction energy between the respective blade platforms and the adjoining damping element. This dampens the vibrations and leads overall to a reduced vibration load on the blade arrangement.
U.S. Pat. No. 4,917,574, for example, discloses such a blade arrangement with damping elements. In this arrangement, the blade platforms of adjacent blades form recesses with their underside, into which recesses spherical bodies are pressed as damping elements by the centrifugal force during rotation.
A further possibility of configuring the damping elements is to design them as bar-shaped elements which have a round cross section and are arranged parallel to the rotor axis between adjacent blades. In this case, the arrangement may be made, for example, in a corresponding lateral recess of the blade root or the blade platform of one of the adjacent blades. Such an arrangement is dealt with, for example, in A. J. Scalzo, Journal of Engineering for Gas Turbines and Power, Vol. 114, April 1992, on pages 289 and 290. This form (used frequently) of the damping elements having a circular cross section additionally seals off the gas flow of a gas turbine from the rotor and is therefore also designated as xe2x80x9cseal-pin damperxe2x80x9d. However, a disadvantage of these damping elements consists in the fact that the damping element can jam under certain conditions. Relative movements are thereby prevented, as a result of which large stresses are produced at the transition points from the damping element to the blades. These stresses lead to premature material fatigue and can promote the formation of cracks in the blades. Furthermore, these damping elements having a circular cross-sectional shape do not act in the same way in the case of all the vibrations of a blade arrangement which occur, so that certain vibration states may occur in a virtually undamped manner. In particular, in the case of these damping elements having a circular cross-sectional shape, it may happen that no relative movement occurs between the contact surfaces or the damping elements roll on the contact surface instead of performing a sliding movement.
A further blade arrangement with damping elements is described, for example, in U.S. Pat. No. 5,156,528. In this arrangement, marginal regions of adjacent blade platforms opposite one another form a recess or guide which narrows in the radial direction and into which the damping element is pressed by the centrifugal force. In this case, the damping element is designed with a wedge-shaped cross section, the wedge angle corresponding to the angle of the V-shaped recess formed by the two marginal regions of the blade platforms. With this wedge-shaped configuration of the damping elements, vibration modes of the blade arrangement which are different from those which are effectively influenced by the damping elements described above can in turn be efficiently damped. In particular, these wedge-shaped vibration elements are not suitable for the damping of equiphase vibration modes. Furthermore, there is the problem with these damping elements that they can tilt during use under certain conditions, as a result of which the damping effect is greatly reduced.
The object of the present invention consists in specifying a blade arrangement with damping elements and also a method of damping vibrations of a blade arrangement, with which blade arrangement and method good damping of a multiplicity of different vibration states can be achieved.
The blade arrangement with damping elements comprises a rotor and blades arranged on the circumference of the rotor. Damping elements are arranged between the blades and are brought into contact with the blades due to the centrifugal force, acting in the radial direction, during rotation of the rotor about the rotor axis. The blade arrangement is characterized in that a plurality of damping elements are arranged one behind the other in the circumferential direction of the rotor at least between two adjacent blades. These damping elements are configured and arranged in such a way that, during rotation of the rotor, the damping elements arranged one behind the other come into contact with one another via one or more contact surfaces, and a first damping element of the damping elements arranged one behind the other comes into contact with a first friction surface of one of the adjacent blades and a second damping element of the damping elements arranged one behind the other comes into contact with a second friction surface of the other adjacent blade.
In the present arrangement, in contrast to the known arrangements of the prior art, one or more further surfaces, the contact surfaces between the two or more damping elements, are thus available for the conversion of kinetic vibration energy into friction energy. Furthermore, due to this/these additional contact surface/surfaces, the risk of seizing of the damping elements, as can occur under certain vibration conditions in the case of the damping elements of the prior art having a circular cross-sectional shape, is reduced. The present arrangement offers in particular the possibility of designing the two or more damping elements in forms differing from one another in order to be able to optimally adapt them to the respective damping requirements. In this case, there are no limits to the diversity of forms, as long as the mutual friction contacts and the friction contacts with the blades or blade platforms can be maintained during operation.
The mass center of the group of damping elements arranged one behind the other may be selected in such a way that it does not lie symmetrically between the two adjacent blades or friction surfaces in the circumferential direction of the rotor. As a result, the load can be distributed nonuniformly over the damping elementsxe2x80x94in particular when using two damping elements arranged one behind the other. The asymmetry may be specifically set by a different geometrical configuration or by different masses of the two damping elements. Due to the multiplicity of possible combinations, the groups of damping elements can be optimally configured for each application. In particular, the suitable selection of the friction or contact surfaces, the mass and the position of the mass center can ensure that the damping elements do not seize.
In this case, the damping elements should also have a high stiffness/weight ratio. This may also be achieved by a hollow form of these elements.
The damping elements of a group may be made of different materials. Thus, for example, cobalt may be selected as the basic material of one of two damping elements, and nickel may be selected as the basic material of the other damping element. This permits different coefficients of friction at the respective friction surfaces with the blades, so that, due to the material selection, further adaptability is available for achieving optimal vibration damping.
The damping elements in this case, just as in the known arrangements of the prior art, are pressed against the underside of the blade platforms by the centrifugal force during the rotation of the rotor. For this purpose, the blade platforms should be suitably shaped or should form grooves at their undersides in adaptation to the form of the damping elements. However, it goes without saying that, in addition to the blade platforms, other regions of the blade root may also be designed for accommodating the damping elements by suitable shaping. In the state of rest of the rotor, the damping elements can also be held by suitable retaining systems.
In a preferred embodiment of the present blade arrangement, a combination of a bar-shaped damping element having a circular cross section and a wedge-shaped damping element, i.e. a bar-shaped element having a wedge-shaped cross section, is used. A multiplicity of different vibration states can be effectively damped by this combination of damping elements having a different effect. Whereas primarily equiphase vibrations are effectively damped with the bar-shaped damping element having a circular cross section, the wedge-shaped damping element acts primarily on vibrations which do not occur in equiphase.
Furthermore, due to the contact surface additionally produced between the damping elements for absorbing vibration energy, the problem of seizing which occurs when using only a bar-shaped damping element having a circular cross section can be avoided. This prolongs the service life of the blade arrangement.
More than two damping elements may of course also be arranged one behind the other. Thus, for example, three damping elements may be used, of which one has a circular cross section and the other two have a wedge-shaped cross sectionxe2x80x94or vice versa.
The group according to the invention of damping elements arranged one behind the other can be used only between individual blades or also between all the adjacent blades of the blading. Recently, it has also been found that fluttering can be reduced or avoided by mistuning of the dampers. In this case, the possibility of an asymmetrical damper configuration of the present invention offers distinct advantages. Thus, for example, the damping elements, arranged one behind the other, of a group may be made of different materials and/or may have different geometrical forms, this pattern of materials or forms repeating itself in a transposed manner over the entire blading. Likewise, for example, the relative position of a damping element having a wedge-shaped cross section to a damping element having a circular cross section can be transposed from blade to blade in order to achieve the desired mistuning.
Furthermore, two or more groups of the damping elements arranged one behind the other can be arranged between respectively adjacent blades in an identical or different configuration over the axial extent of the blades. As a result, it is possible to effectively dampen the most diverse vibration forms. The damper configurations of the individual groups are in each case optimized in form and/or mass ratio and/or geometrical dimensions in accordance with the vibration form to be damped.